1. Field of the Invention
This invention relates to control circuits for current controllers in general and more particularly to a control system for the shunt field and armature windings of a direct current motor.
2. Description of the Prior Art
Various systems for controlling the speed of a direct current electric motor have been disclosed in the prior art. Probably the most simple method of speed control is the use of an adjustable resistor in series with the armature circuit. The resistance is increased for starting or for short-time or intermittent slowdowns in its most common usage, but this control has the disadvantage of power loss in the resistor which decreases the system efficiency. A second type of speed control involves a constant armature voltage and a variable voltage applied to the field or an adjustable resistor in series with the field to achieve control over a speed range of approximately four or five to one. The maximum torque is limited by the permissible armature current and the maximum flux which, in turn, is limited by magnetic saturation or by heating of the field winding. This type of control has the disadvantage of slow response when the field current polarity is reversed to change the direction of rotation since larger horsepower motors generally have relatively large field time constants.
Another form of motor speed control involves the use of constant armature current and a variable field excitation. In addition to the disadvantage of the slow response time of the field, this type of control is less common than other controls since constant voltage sources are more readily available than constant current sources. Still another form of speed control is the constant field current and controlled armature voltage type. The armature may be supplied from a controlled rectifier voltage supply or a separately excited direct current generator which is commonly known as a Ward Leonard system.
In another form of speed control, both the armature and the field supply currents may be varied to control speed in response to a control circuit which defines a relationship between these two currents as a function of the speed error signal. For example, the field current may be controlled to reach its maximum value when the armature current has reached approximately one half of its maximum value. If the field controller is bidirectional and the armature controller is unidirectional relatively economical four quadrant (positive and negative values of speed plotted against positive and negative values of torque on coordinate axes) motor control system can be defined which responds to a speed error signal.